FIGS. 4(a) and 4(b) are sectional views showing the structure of a conventional semiconductor device and a method for manufacturing it. Referring to FIG. 4(a), reference numeral 31 designates a substrate on which a semiconductor film 32 is formed. An electrically conductive material is used for the substrate 31 because it serves as an electrode of the semiconductor device when, for example, the semiconductor device is a solar cell.
Referring to FIG. 4(b), the semiconductor film 32 having a thickness of, for example, 30 to 50 microns is formed on the substrate 31 having a thickness of, for example, 0.2 to 0.5 millimeter, as shown in FIG. 4(a). Then, a pn junction and an electrode are formed on the semiconductor film 32 and the semiconductor device is completed.
When polycrystalline silicon is used as the material of the semiconductor film 32, the process for forming the semiconductor film 32 generally requires a high temperature. For example, with silicon, a temperature of at least 600.degree. C. is necessary to form a polycrystalline film by chemical vapor deposition (CVD). If the film has to be formed in a short time, a still higher temperature is necessary. In addition, when the semiconductor film 32 is recrystallized, since the semiconductor film has to be temporarily heated to a melting point of silicon, a still higher processing temperature is used. The substrate 31 must bear up against such a high temperature.
Crystalline silicon, quartz, carbon, ceramics, and the likes are examples of materials capable of bearing up against high temperatures. However, since the substrate 31 is an electrode, it has to be electrically conductive, such as crystalline silicon, carbon, a conductive ceramic, or the like. However, these materials are comparatively expensive. If they are used as the substrate, cost is increased which is contrary to the objective of reducing semiconductor volume and cost.
When a material different from that of the semiconductor film 32 is used for the substrate, when the semiconductor film 32 is formed or during another high temperature process for forming the semiconductor device, the substrate material or a substance contained in the substrate diffuses into the semiconductor film 32 from the substrate 31 as an impurity. Especially, metallic impurities, such as iron, copper, and calcium, cause problems. More specifically, almost all of these impurities act as trapping centers for minority carriers or cause crystalline defects which adversely affect the semiconductor device. Even if the same material as the semiconductor film 32 is used for the substrate, if a semiconductor material having low purity is used for the substrate 31 in order to reduce cost, impurities will also diffuse into the semiconductor film 31.
FIGS. 5(a)-5(d) are sectional views showing manufacturing steps for a structure in which problems in the prior art are solved. In FIG. 5(d), the same reference numerals as in FIG. 4(b) designate the same or corresponding parts. A barrier layer 34 having a thickness of approximately 1 to 2 microns and formed of, for example, silicon dioxide, silicon nitride, or laminated layers of them is formed on the substrate 31 shown in FIG. 5(a) by a CVD method. Then, since the silicon dioxide film or the silicon nitride film is an insulating film, openings 35 are formed by etching desired parts of the barrier layer 34 using photolithography, as shown in FIG. 5(c), to electrically connect the semiconductor film 32 to the substrate 31. Thereafter, the semiconductor film 32 is formed using the same step as in FIG. 4(b), now illustrated in FIG. 5(d). During the high temperature process for forming the semiconductor film 32, the barrier layer prevents harmful impurities from diffusing from the substrate 31 into the semiconductor film 32.
However, since the semiconductor film 32 is directly in contact with the substrate 31 in the openings 35 in the barrier layer 34, impurities in the substrate 31 diffuse into the semiconductor film 32 through the openings as in the conventional example shown in FIG. 4(b). Therefore, even if contact between the semiconductor film 32 and the substrate 31 is limited to the openings of the barrier layer 34 and the degree of mixing of the impurity is reduced because the contact; area is reduced, the problem is not completely solved.
The conventional semiconductor device and manufacturing method described above do not completely prevent harmful impurities from diffusing from the substrate into the semiconductor film. However, if a substrate formed of a material having no such harmful impurity is used, the substrate is expensive and the semiconductor device cannot be manufactured at low cost.